EUV light source components and methods for producing, using and refurbishing same

ABSTRACT

A method is disclosed for in-situ monitoring of an EUV mirror to determine a degree of optical degradation. The method may comprise the steps/acts of irradiating at least a portion of the mirror with light having a wavelength outside the EUV spectrum, measuring at least a portion of the light after the light has reflected from the mirror, and using the measurement and a pre-determined relationship between mirror degradation and light reflectivity to estimate a degree of multi-layer mirror degradation. Also disclosed is a method for preparing a near-normal incidence, EUV mirror which may comprise the steps/acts of providing a metallic substrate, diamond turning a surface of the substrate, depositing at least one intermediate material overlying the surface using a physical vapor deposition technique, and depositing a multi-layer mirror coating overlying the intermediate material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/004,871, filed on Dec. 20, 2007, now U.S. Pat. No. 7,960,701 entitledEUV LIGHT SOURCE COMPONENTS AND METHODS FOR PRODUCING, USING ANDREFURBISHING SAME, the entire contents of which are hereby incorporatedby reference herein.

The present application is related to U.S. patent application Ser. No.11/827,803 filed on Jul. 13, 2007, now U.S. Pat. No. 7,898,947, issuedon Mar. 1, 2011, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE HAVINGA DROPLET STREAM PRODUCED USING A MODULATED DISTURBANCE WAVE, U.S.patent application Ser. No. 11/358,988 filed on Feb. 21, 2006, andpublished on Nov. 16, 2006, as US2006/0255298-A1, entitled LASERPRODUCED PLASMA EUV LIGHT SOURCE WITH PRE-PULSE, U.S. patent applicationSer. No. 11/067,124 filed on Feb. 25, 2005, now U.S. Pat. No. 7,405,416,issued on Jul. 29, 2008, entitled METHOD AND APPARATUS FOR EUV PLASMASOURCE TARGET DELIVERY, U.S. patent application Ser. No. 11/174,443filed on Jun. 29, 2005, now U.S. Pat. No. 7,372,056, issued on May 13,2008, entitled LPP EUV PLASMA SOURCE MATERIAL TARGET DELIVERY SYSTEM,U.S. patent application Ser. No. 11/358,983, filed on Feb. 21, 2006, nowU.S. Pat. No. 7,378,673, issued on May 27, 2008, entitled SOURCEMATERIAL DISPENSER FOR EUV LIGHT SOURCE, U.S. patent application Ser.No. 11/358,992 filed on Feb. 21, 2006, now U.S. Pat. 7,598,509, issuedon Oct. 6, 2009, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE, U.S.patent application Ser. No. 11/174,299 filed on Jun. 29, 2005, now U.S.Pat. No. 7,439,530, issued on Oct. 21, 2008, and entitled, LPP EUV LIGHTSOURCE DRIVE LASER SYSTEM, U.S. patent application Ser. No. 11/406,216filed on Apr. 17, 2006, now U.S. Pat. No. 7,465,946, issued on Dec. 16,2008, entitled ALTERNATIVE FUELS FOR EUV LIGHT SOURCE, U.S. patentapplication Ser. No. 11/580,414 filed on Oct. 13, 2006, now U.S. Pat.No. 7,491,954, issued on Feb. 17, 2009, entitled, DRIVE LASER DELIVERYSYSTEMS FOR EUV LIGHT SOURCE, U.S. patent application Ser. No.11/644,153 filed on Dec. 22, 2006, and published on Jun. 26, 2008, asUS2008/0149862-A1, entitled, LASER PRODUCED PLASMA EUV LIGHT SOURCE,U.S. patent application Ser. No. 11/505,177 filed on Aug. 16, 2006, nowU.S. Pat. No. 7,843,632, issued on Nov. 30, 2010, entitled EUV OPTICS,U.S. patent application Ser. No. 11/452,558 filed on Jun. 14, 2006, nowU.S. Pat. 7,518,787, issued on Apr. 14, 2009, entitled DRIVE LASER FOREUV LIGHT SOURCE, U.S. Pat. No. 6,928,093, issued to Webb, et al. onAug. 9, 2005, entitled LONG DELAY AND HIGH TIS PULSE STRETCHER, U.S.application Ser. No. 11/394,512, filed on Mar. 31, 2006, and titledCONFOCAL PULSE STRETCHER, U.S. patent application Ser. No. 11/138,001,filed on May 26, 2005, and published on Nov. 24, 2005, asUS2005/0259709-A1, and titled SYSTEMS AND METHODS FOR IMPLEMENTING ANINTERACTION BETWEEN A LASER SHAPED AS A LINE BEAM AND A FILM DEPOSITEDON A SUBSTRATE; and U.S. patent application Ser. No. 10/141,216, filedon May 7, 2002, now U.S. Pat. No. 6,693,939, and titled, LASERLITHOGRAPHY LIGHT SOURCE WITH BEAM DELIVERY, U.S. Pat. No. 6,625,191issued to Knowles et al on Sep. 23, 2003, entitled VERY NARROW BAND, TWOCHAMBER, HIGH REP RATE GAS DISCHARGE LASER SYSTEM, U.S. patentapplication Ser. No. 10/012,002, U.S. Pat. No. 6,549,551 issued to Nesset al on Apr. 15, 2003, entitled INJECTION SEEDED LASER WITH PRECISETIMING CONTROL, U.S. patent application Ser. No. 09/848,043; U.S. Pat.No. 6,567,450 issued to Myers et al on May 20, 2003 entitled VERY NARROWBAND, TWO CHAMBER, HIGH REP RATE GAS DISCHARGE LASER SYSTEM, U.S. patentapplication Ser. No. 09/943,343, U.S. patent application Ser. No.11/509,925 filed on Aug. 25, 2006, now U.S. Pat. No. 7,476,886, issuedon Jan. 13, 2009, entitled SOURCE MATERIAL COLLECTION UNIT FOR A LASERPRODUCED PLASMA EUV LIGHT SOURCE, the entire contents of each of whichare hereby incorporated by reference herein.

FIELD

The present application relates to extreme ultraviolet (“EUV”) lightsources providing EUV light from a plasma created from a source materialand collected and directed to a focus for utilization outside of the EUVlight source chamber, e.g., for semiconductor integrated circuitmanufacturing photolithography e.g., at wavelengths of around 50 nm andbelow.

BACKGROUND

EUV light, e.g., electromagnetic radiation in the EUV spectrum (i.e.having wavelengths of about 5-100 nm) may be useful in photolithographyprocesses to produce extremely small features, e.g. sub-32 nm features,in semiconductor substrates such as silicon wafers.

Methods to produce EUV light include, but are not necessarily limitedto, converting a material into a plasma state that has one or moreelements, e.g., xenon, lithium or tin, indium, antimony, tellurium,aluminum, etc., with one or more emission line(s) in the EUV spectrum.In one such method, often termed laser produced plasma (“LPP”), a plasmacan be produced by irradiating a target material, such as a droplet,stream or cluster of material having the line-emitting element, with alaser beam. Another method involves disposing the line-emitting elementbetween two electrodes. In this method, often termed discharge producedplasma (“DPP”), a plasma can be produced by creating an electricaldischarge between the electrodes.

Heretofore, various systems in which a line-emitting element ispresented for irradiation/electric discharge have been disclosed. Manydiverse forms and states have been attempted, to include, presenting theelement in pure form, e.g., pure metal, presenting the element as acompound, e.g., a salt, or as an alloy, e.g. with some other metal, orin a solution, e.g., dissolved in a solvent such as water. Moreover,systems have been disclosed in which the line-emitting substance ispresented as a liquid, including relatively volatile liquids, a gas, avapor and/or a solid, and can be in the form of a droplet, stream,moving tape, aerosol, particles in a liquid stream, particles in adroplet stream, gas jet, etc.

For these processes, the plasma is typically produced in a sealedvessel, e.g., vacuum chamber, and monitored using various types ofmetrology equipment. A typical EUV light source may also include one ormore EUV mirrors e.g., a substrate covered with a graded, multi-layercoating such as Mo/Si. One or more of these mirrors are then disposed inthe sealed vessel, distanced from the irradiation site, and oriented todirect EUV light emitted from the plasma to an EUV light source output.In general, these EUV mirrors may be either near-normal incidence typemirrors or grazing incidence type mirrors. By way of example, for an LPPsetup, the mirror may be in the form of an ellipsoidal, e.g. a prolatespheroid having a circular cross section normal to a line passingthrough its loci near-normal incidence type, with an aperture to allowthe laser light to pass through and reach the irradiation site. Withthis arrangement, the irradiation site may be positioned at or near afirst focus of the ellipsoid and the light source output may bepositioned at, near or downstream of the second ellipsoid focus.

In addition to generating EUV radiation, these plasma processesdescribed above may also generate undesirable by-products in the plasmachamber which can include out-of-band radiation, high energy ions anddebris, e.g., atoms and/or clumps/micro-droplets of the target material.These plasma formation by-products can potentially heat, damage orreduce the operational efficiency of the various plasma chamber opticalelements including, but not limited to, collector mirrors includingmulti-layer mirror coatings (MLM's) capable of EUV reflection atnear-normal incidence and/or grazing incidence, the surfaces ofmetrology detectors, windows used to image the plasma formation process,and the laser input window. The heat, high energy ions and/or debris maybe damaging to the optical elements in a number of ways, includingcoating them with materials which reduce light transmission, penetratinginto them and, e.g., damaging structural integrity and/or opticalproperties, e.g., the ability of a mirror to reflect light at such shortwavelengths, corroding, roughening or eroding them and/or diffusing intothem.

Accessing contaminated or damaged optical elements in the plasma chamberfor the purpose of cleaning or replacing the elements can be expensive,labor intensive and time-consuming. In particular, these systemstypically require a rather complicated and time consuming purging andvacuum pump-down of the plasma chamber prior to a re-start after theplasma chamber has been opened. This lengthy process can adverselyaffect production schedules and decrease the overall efficiency of lightsources for which it is typically desirable to operate with little or nodowntime.

For some target materials, e.g., tin, it may be desirable to introducean etchant, e.g., HBr or some other halogen-containing compound, or Hradicals, into the plasma chamber to etch material, e.g. debris that hasdeposited on the optical elements. This etchant may be present duringlight source operation, during periods of non-operation, or both. It isfurther contemplated that the affected surfaces of one or more elementsmay be heated to initiate reaction and/or increase the chemical reactionrate of the etchant and/or to maintain the etching rate at a certainlevel. For other target materials, e.g., lithium, it may be desirable toheat the affected surfaces where lithium debris has deposited to atemperature sufficient vaporize at least a portion of the depositedmaterial, e.g. a temperature in the range of about 400 to 550 degrees C.to vaporize Li from the shield surface, with or without the use of anetchant.

One way to reduce the influence of debris is to move the collectormirror further away from the irradiation site. This, in turn, impliesthe use of a larger collector mirror to collect the same amount oflight. The performance of a collector mirror, e.g., the ability toaccurately direct as much in-band light as possible to, e.g., a focalpoint, depends of the figure and surface finish, e.g., roughness of thecollector. As one might expect, it becomes more and moredifficult/expensive to produce a suitable figure and surface finish asthe size of the collector mirror grows. For this environment, EUV mirrorsubstrate considerations may include one or more of the following:vacuum compatibility, mechanical strength, e.g. high temperaturestrength, high thermal conductivity, low thermal expansion, dimensionalstability, and ease of producing a suitable figure and finish.

Many factors may affect the in-band output intensity (and angularintensity distribution) from an EUV light source and these factors maychange over the lifetime of the light source. For example, in an LPPsystem, changes in collector reflectivity, target size, laser pulseenergy and duration and/or coupling of laser pulse and target material,e.g, as a function of steering and focusing may affect in-band EUVoutput intensity. Thus, it may be desirable to determine whichcomponent/sub-systems are adversely affecting in-band EUV outputintensity so that the problem can be remedied. If possible, it may bedesirable to diagnose the performance of each component/sub-system whilethey are in position in the light source (i.e. in-situ) and/or while theEUV light source is operating.

With the above in mind, applicants disclose EUV light source componentsand methods for producing, using and refurbishing EUV light sourcecomponents.

SUMMARY

In a first aspect of an embodiment of the present patent application, amethod for in-situ monitoring of an EUV mirror to determine a degree ofoptical degradation may comprise the step/act of irradiating at least aportion of the mirror with light having a wavelength outside the EUVspectrum while the EUV mirror is in an operational position in aphotolithography apparatus. The method may further comprise thesteps/acts of measuring at least a portion of the light after the lighthas reflected from the mirror and using the measurement and apre-determined relationship between mirror degradation and lightreflectivity to estimate a degree of multi-layer mirror degradation.

In one implementation of this method, the mirror may be operationalypositioned in an EUV light source portion of the photolithographyapparatus and in a particular implementation, the mirror may be anear-normal incidence multi-layer mirror. In one embodiment, theirradiating act may be performed with a point source of visible light,for example, the point source may be a light emitting diode or a lightsource in combination with an aperture, e.g. a relatively small apertureto define a small region of light emission.

For this method, the mirror may have an ellipsoidal shape defining afirst focus and a second focus, and the irradiating step/act may beperformed with a point source of visible light positioned at the firstfocus and the measuring step/act may be performed with a detectorpositioned at a location closer to the second focus than the firstfocus.

For example, the irradiating step/act may be performed with a pointsource positioned at the first focus generating a cone of reflectedlight having an apex at the second focus, and diffuse reflected light.For this arrangement, the measuring step/act may be performed with adetector positioned at a distance from the second focus to detectdiffuse reflected light. In one aspect of a particular implementation,the irradiating step I act may be performed with a laser beam and themeasuring step/act may be performed with a detector positioned to detectdiffuse reflected light.

In another aspect of an embodiment of the present patent application, asystem for in-situ monitoring of an EUV mirror to determine a degree ofoptical degradation may comprise a light source irradiating at least aportion of the mirror with light having a wavelength in the visiblespectrum while the EUV mirror is in an operational position in aphotolithography apparatus, and a detector measuring an intensity of atleast a portion of the light after the light has reflected from themirror, the detector generating an output signal for use in estimating adegree of multi-layer mirror degradation.

In still another aspect of an embodiment of the present patentapplication, a metrology device for measuring a characteristic of EUVradiation may comprise a detecting element and a filter comprisingsilicon nitride. In one arrangement, the detecting element may be afluorescent converter and the measured characteristic may be angularintensity distribution. In another arrangement, the detecting elementmay be a photodiode and the measured characteristic may be intensity.

In one arrangement of this aspect, the filter may further comprise Ru,e.g. a layer of Ru, and in a particular arrangement of this aspect, thefilter may further comprise Ru and Zr. In another arrangement of thisaspect, the filter may further comprise a plurality of Ru layers and aplurality of silicon nitride layers. In yet another arrangement of thisaspect, the filter may further comprise Pd, e.g., a layer of Pd, and ina particular arrangement of this aspect, the filter may further comprisePd and Zr, In another arrangement of this aspect, the filter may furthercomprise a plurality of Pd layers and a plurality of silicon nitridelayers.

In still another aspect of an embodiment of the present patentapplication, a metrology device for measuring an angular intensitydistribution of EUV light may comprise a fluorescent converter and afilter comprising uranium, In one implementation of this aspect, thefilter may further comprise Ru.

In another aspect of an embodiment of the present patent application, ametrology device, e.g. fluorescent converter or photodiode, formeasuring a characteristic of EUV radiation may comprise a detectingelement and a filter comprising a transmissive multilayer coating havinga plurality of bi-layers, each bi-layer having a relatively lowrefractive index material and a relatively high refractive indexmaterial. For example, the coating may be transmissive multilayercoating comprising a plurality of Mo layers and a plurality of Si layersor a transmissive multilayer coating comprising a plurality of Zr layersand a plurality of Si layers.

In another aspect of an embodiment of the present patent application, ametrology device, e.g. fluorescent converter or photodiode, formeasuring a characteristic of EUV radiation may comprise a detectingelement and a filter comprising uranium and Ru.

In another aspect, a device for removing debris from an EUV light sourcecollector mirror, the debris generated by plasma formation, thecollector mirror having a first side covered with a multi-layernear-normal incidence reflective coating and an opposed side, maycomprise: a conductive coating deposited to overlay at least a portionof the opposed side; and a system generating electrical currents in thecoating to heat the collector mirror. The conductive coating may be, butis not necessarily limited to a vacuum deposited coating, aflame-sprayed coating, an electroplated coating or a combinationthereof,

In one embodiment, the coating may be configured to heat a first zone ofthe collector mirror to a first temperature, T₁, to remove debris fromthe first zone and heat a second zone of the collector mirror to asecond temperature, T₂, to remove debris from the second zone, withT₁≠T₂. In a particular embodiment, the first zone may have a differentcoating coverage by area than the second zone. In another particularembodiment, the first zone may have a different coating thickness thanthe second zone. In another particular embodiment, the first zone mayhave a different coating conductivity than the second zone. The aboveimplies that there could be three or more zones at differenttemperatures, e.g., more than just two zones. In one implementation, thesystem may be configured to generate electro-magnetic radiation and thesystem may deliver an electro-magnetic radiation power, P₁, to the firstzone of the collector mirror, and an electro-magnetic radiation power,P₂, to the second zone of the collector mirror, with P₁≠P₂.

In another aspect of the present patent application, an EUV collectormirror device having a surface exposed to debris generated by plasmaformation, may comprise a substrate covered with a multi-layernear-normal incidence reflective coating, the substrate made of amaterial doped with a conductive material; and a system generatingelectrical currents in the substrate to heat the collector mirror.

In one embodiment of this aspect, the coating may be configured to heata first zone of the collector mirror to a first temperature, T₁, toremove debris from the first zone and heat a second zone of thecollector mirror to a second temperature, T₂, to remove debris from thesecond zone, with T₁≠T₂. In a particular implementation, the first zonemay have a different substrate conductivity than the second zone. Inanother implementation, the system may be configured to generateelectro-magnetic radiation and the system may deliver anelectro-magnetic radiation power, P₁, to the first zone of the collectormirror, and an electro-magnetic radiation power, P₂, to the second zoneof the collector mirror, with P₁≠P₂.

In another aspect of the present patent application, a method forpreparing a near-normal incidence, EUV mirror may comprise thesteps/acts of providing a substrate; diamond turning a surface of thesubstrate; depositing at least one intermediate material overlying thesurface using physical vapor deposition; and depositing a multi-layermirror coating overlying the intermediate material.

For example, the multilayer mirror coating may comprise alternatinglayers of Mo and Si.

For this aspect, the substrate may be selected from the group ofmetallic materials consisting of invar, covar, monel, hastelloy, nickel,inconel, titanium, nickel-phosphite plated/coated aluminum,nickel-phosphite plated/coated invar, nickel-phosphite plated/coatedcovar or the substrate may be a semi-conductor material , e.g. single-or multi-crystalline silicon, In some arrangements, the mirror may be anellipsoidal mirror having a diameter greater than 500 mm.

In one embodiment, the intermediate material may comprises an etch stopmaterial having a substantially different etch sensitivity than themulti-layer mirror coating for at least one etchant, and in particularembodiments, the etch stop material may be selected from the group ofmaterials consisting of Si, B₄C, an oxide, SiC and Cr. In some cases,the etch stop layer may have a thickness in the range of 3 nm to 100 nm.

In one embodiment, the intermediate material may comprise a barriermaterial substantially reducing diffusion of the metallic substrate intothe multi-layer mirror coating, and in particular embodiments, thebarrier material may be selected from the group of materials consistingof ZrN, Zr, MoSi₂, Si₃N₄, B₄C, SiC and Cr.

In one embodiment, the intermediate material may comprise a smoothingmaterial, and in particular embodiments, the smoothing material may beselected from the group of materials consisting of Si, C, Si₃N₄, B₄C,SiC, ZrN, Zr and Cr. In some implementations, the smoothing material maybe deposited using highly energetic deposition conditions, for example,the deposition conditions include substrate heating and/or thedeposition conditions include increasing particle energy duringdeposition.

In some cases, the smoothing layer may overlay and contact the metallicsubstrate. In one embodiment, the smoothing layer may have a thicknessin the range of 3 nm to 100 nm. In a particular implementation, thesmoothing layer may comprise an amorphous material.

In one implementation, the depositing step/act may be performed using aphysical vapor deposition technique selected from the group oftechniques consisting of ion beam sputter deposition, electron beamphysical vapor deposition magnetron sputtering and combinations thereof.

In another aspect of the present patent application, a method forrefurbishing a near-normal incidence, EUV mirror may comprise thesteps/acts of providing an EUV mirror having a substrate, at least oneintermediate material overlying the substrate and a multi-layer mirrorcoating overlying the intermediate material; removing the multi-layermirror coating to produce an exposed surface; and thereafter chemicallypolishing the exposed surface; depositing a smoothing material; anddepositing a multi-layer mirror coating overlying the smoothingmaterial.

In one implementation of this aspect, the intermediate layer may have athickness in the range of 5 μm to 15 μm and the removing step/act mayuse diamond turning to remove the multi-layer mirror coating.

In particular implementations, the smoothing material may be selectedfrom the group of materials consisting of ZrN, Zr, MoSi₂, Si₃N₄, B₄C,SiC and Cr.

In a particular implementation, the multi-layer mirror coating mayoverlay and contact the smoothing material.

In a particular implementation a first intermediate layer, the etch stoplayer, e.g. a Cr layer or a TiO₂ layer, may be covered by a secondintermediate layer, a smoothing layer or diffusion barrier layer, e.g.,ZrN, Zr, Si, C, Si₃N₄, B₄C, SiC, or MoSi₂ in order to reduce a surfaceroughening effect that may be caused by the deposition of the etch stoplayer.

In one implementation, the removing step/act may use chemical etching toremove the multi-layer mirror coating and in a particularimplementation, the intermediate layer may have a thickness in the rangeof 5 nm to 20 nm and the removing step/act may use chemical etching toremove the multi-layer mirror coating.

In another aspect of the present patent application, a method forproducing EUV light may comprising the acts of providing an EUV mirrorhaving a substrate, a first multi-layer coating stack, a stop layeroverlying the first multi-layer coating stack and a second multi-layercoating stack overlying the stop layer; using the mirror to reflect EUVlight produced by an EUV light emitting plasma, the plasma generatingdebris which degrades the second multi-layer coating stack; andthereafter; etching the mirror to expose at least a portion of the stoplayer; and thereafter; using the mirror to reflect EUV light produced byan EUV light emitting plasma.

In one implementation of this aspect, the stop layer may comprise amaterial selected from the group of materials consisting of ZrN, Zr,Si₃N₄, SiB₆, SiC, C, Cr, B₄C, Mo₂C, SiO₂, ZrB₂, YB₆ and MoSi₂, and theetching step may employ an etchant selected from the group of materialsconsisting of Cl₂, HCl, CF₄, and mixtures thereof.

In one embodiment of this method, the second multi-layer coating stackmay comprise a plurality of bi-layers, each bi-layer having a layer ofMo and a layer of Si, and in a particular embodiment, the secondmulti-layer coating stack may comprise a plurality of Mo layers, aplurality of Si layers and a plurality of diffusion barrier layersseparating Mo layers from Si layers,

In one arrangement, the second multi-layer coating stack may comprisemore than forty bi-layers.

In some cases, the stop layer may have a thickness selected to maintainthe periodicity of the mirror from the second multi-layer coating stackto the first multi-layer coating stack.

In a particular implementation of this aspect, the stop layer may be afirst stop layer and the mirror may comprise a second stop layeroverlying the second multi-layer coating stack and a third multi-layercoating stack overlying the second stop layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified schematic view of a laser-produced plasma EUVlight source according to an aspect of the present disclosure;

FIG. 2 shows a schematic view of portions of an LPP EUV light sourcehaving an apparatus for in-situ monitoring of an ellipsoidal EUV mirrorto determine a degree of optical degradation;

FIG. 3 shows a schematic view of portions of an LPP EUV light sourcehaving a different embodiment of an apparatus for in-situ monitoring ofan ellipsoidal EUV mirror to determine a degree of optical degradation;

FIG. 4 shows a schematic view of portions of an LPP EUV light sourcehaving a different embodiment of an apparatus for in-situ monitoring ofan ellipsoidal EUV mirror to determine a degree of optical degradation;

FIG. 5 shows a schematic view of portions of an LPP EUV light sourcehaving a different embodiment of an apparatus for in-situ monitoring ofan ellipsoidal EUV mirror to determine a degree of optical degradation;

FIG. 6 illustrates a portion of a metrology device for measuring acharacteristic of EUV radiation having a detecting element and anarrowband EUV transmission filter;

FIG. 7A shows calculated plots of transmission intensity (normalized)versus wavelength in nanometers for various filter materials;

FIGS. 7B, 7C and 7D show calculated plots of transmission intensityversus wavelength in nanometers for de-enriched uranium filters havingvarious thicknesses;

FIG. 7E shows a calculated plot of transmission intensity versuswavelength in nanometers for a filter having about 0.2 μm thick uraniumlayer and about 50 nm thick Ru layer;

FIGS. 8A, 8B and 8C illustrate three embodiments of backside heaters foran EUV reflective mirror;

FIG. 9A shows a sectional view of a near-normal incidence EUV collectormirror having a substrate, intermediate layer, and multi-layer minorcoating;

FIG. 9B shows a sectional view of a mirror, e.g., a near-normalincidence EUV collector mirror, having a substrate, first intermediatelayer, second intermediate layer and multi-layer mirror coating;

FIG. 10 shows a sectional view of a mirror, e.g., a near-normalincidence EUV collector mirror, having a substrate, five multi-layercoating stacks and four stop layers separating the multi-layer coatingstacks; and

FIG. 11 shows a sectional view illustrating a minor, e.g., a near-normalincidence EUV collector mirror, having a substrate, a plurality ofmulti-layer coating stacks separated by respective stop layers, eachmulti-layer coating stack having a plurality of relatively highrefractive index layers, a plurality of relatively low refractive indexlayers and a plurality of diffusion barrier layers separating high indexlayers from the low index layers.

DETAILED DESCRIPTION

With initial reference to FIG. 1 there is shown a schematic view of anEUV light source, e.g., a laser-produced-plasma, EUV light source 20according to one aspect of an embodiment. As shown in FIG. 1, anddescribed in further details below, the LPP light source 20 may includea system 22 for generating a train of light pulses and delivering thelight pulses into a chamber 26. As detailed below, each light pulse maytravel along a beam path from the system 22 and into the chamber 26 toilluminate a respective target droplet at an irradiation region, e.g. ator near a focus 28 of an ellipsoidal mirror.

Suitable lasers for use as the device 22 shown in FIG. 1 may include apulsed laser device, e.g., a pulsed gas discharge CO₂ laser deviceproducing radiation at 9.3 μm or 10.6 μm, e.g., with DC or RFexcitation, operating at relatively high power, e.g., 10 kW or higherand high pulse repetition rate, e.g., 50 kHz or more. In one particularimplementation, the laser may be an axial-flow RF-pumped CO₂ having aMOPA configuration with multiple stages of amplification and having aseed pulse that is initiated by a Q-switched Master Oscillator (MO) withlow energy and high repetition rate, e.g., capable of 100 kHz operation.From the MO, the laser pulse may then be amplified, shaped, steeredand/or focused before entering the LPP chamber. Continuously pumped CO₂amplifiers may be used for the system 22. For example, a suitable CO₂laser device having an oscillator and three amplifiers (O-PA1-PA2-PA3configuration) is disclosed in U.S. patent application Ser. No.11/174,299 filed on Jun. 29, 2005, now U.S. Pat. No. 7,439,530, issuedon Oct. 21, 2008, and entitled, LPP EUV LIGHT SOURCE DRIVE LASER SYSTEM;the entire contents of which have been previously incorporated byreference herein. Alternatively, the laser may be configured as aso-called “self-targeting” laser system in which the droplet serves asone mirror of the optical cavity, In some “self-targeting” arrangements,a master oscillator may not be required. Self targeting laser systemsare disclosed and claimed in U.S. patent application Ser. No. 11/580,414filed on Oct. 13, 2006, now U.S. Pat. No. 7,491,954, issued on Feb. 17,2009, entitled, DRIVE LASER DELIVERY SYSTEMS FOR EMT LIGHT SOURCE; theentire contents of which have been previously incorporated by referenceherein.

Depending on the application, other types of lasers may also besuitable, e.g., an excimer or molecular fluorine laser operating at highpower and high pulse repetition rate. Examples include, a solid statelaser, e.g., having a rod, fiber or disk shaped active media, a MOPAconfigured excimer laser system, e.g., as shown in U.S. Pat. Nos.6,625,191, 6,549,551, and 6,567,450, an excimer laser having one or morechambers, e.g., an oscillator chamber and one or more amplifyingchambers (with the amplifying chambers in parallel or in series), amaster oscillator/power oscillator (MOPO) arrangement, a poweroscillator/power amplifier (POPA) arrangement, or a solid state laserthat seeds one or more excimer or molecular fluorine amplifier oroscillator chambers, may be suitable. Other designs are possible.

As further shown in FIG. 1, the EUV light source 20 may also include atarget material delivery system 24, e.g., delivering droplets of atarget material into the interior of a chamber 26 to the irradiationregion where the droplets will interact with one or more light pulses,e.g., one or more pre-pulses and thereafter one or more main pulses, toultimately produce a plasma and generate an EUV emission. The targetmaterial may include, but is not necessarily limited to, a material thatincludes tin, lithium, xenon or combinations thereof. The EUV emittingelement, e.g., tin, lithium, xenon, etc., may be in the form of liquiddroplets and/or solid particles contained within liquid droplets. Forexample, the element tin may be used as pure tin, as a tin compound,e.g., SnBr₄, SnBr₂, SnH₄, as a tin alloy, e.g., tin-gallium alloys,tin-indium alloys, tin-indium-gallium alloys, or a combination thereof.Depending on the material used, the target material may be presented tothe irradiation region at various temperatures including roomtemperature or near room temperature (e.g., tin alloys, SnBr₄) at anelevated temperature, (e.g., pure tin) or at temperatures below roomtemperature, (e.g., Sna₄), and in some cases, can be relativelyvolatile, e.g., SnBr₄. More details concerning the use of thesematerials in an LPP EUV source is provided in U.S. patent applicationSer. No. 11/406,216 filed on Apr. 17, 2006, now U.S. Pat. No. 7,465,946,issued on Dec. 16, 2008, entitled ALTERNATIVE FUELS FOR EUV LIGHTSOURCE; the contents of which have been previously incorporated byreference herein.

Continuing with FIG. 1, the EUV light source 20 may also include anoptic 30, e.g., a collector mirror in the form of a truncated ellipsoidhaving, e.g., a graded multi-layer coating with alternating layers ofmolybdenum and silicon. FIG. 1 shows that the optic 30 may be formedwith an aperture to allow the light pulses generated by the system 22 topass through and reach the irradiation region. As shown, the optic 30may be, e.g., an ellipsoidal mirror that has a first focus within ornear the irradiation region and a second focus at a so-calledintermediate region 40 where the EUV light may be output from the EUVlight source 20 and input to a device utilizing EUV light, e.g., anintegrated circuit lithography tool (not shown). It is to be appreciatedthat other optics may be used in place of the ellipsoidal mirror forcollecting and directing light to an intermediate location forsubsequent delivery to a device utilizing EUV light, for example theoptic may be parabolic or may be configured to deliver a beam having aring-shaped cross-section to an intermediate location, see e.g. U.S.patent application Ser. No. 11/505,177 filed on Aug. 16, 2006, now U.SPat. No. 7,843,632, issued on Nov. 30, 2010, entitled EUV OPTICS; thecontents of which are hereby incorporated by reference.

Continuing with reference to FIG. 1, the EUV light source 20 may alsoinclude an EUV controller 60, which may also include a firing controlsystem 65 for triggering one or more lamps and/or laser devices in thesystem 22 to thereby generate light pulses for delivery into the chamber26. The EUV light source 20 may also include a droplet positiondetection system which may include one or more droplet imagers 70 thatprovide an output indicative of the position of one or more droplets,e.g., relative to the irradiation region. The imager(s) 70 may providethis output to a droplet position detection feedback system 62, whichcan, e.g., compute a droplet position and/or trajectory, from which adroplet position error can be computed, e.g., on a droplet by dropletbasis or on average. The droplet error may then be provided as an inputto the controller 60, which can, for example, provide a position,direction and/or timing correction signal to the system 22 to control asource timing circuit and/or to control a beam position and shapingsystem, e.g., to change the location and/or focal power of the lightpulses being delivered to the irradiation region in the chamber 26.

The EUV light source 20 may include one or more EUV metrologyinstruments for measuring various properties of the EUV light generatedby the source 20. These properties may include, for example, intensity(e.g., total intensity or intensity within a particular spectral band),spectral bandwidth, polarization, beam position, pointing, etc. For theEUV light source 20, the instrument(s) may be configured to operatewhile the downstream tool, e.g., photolithography scanner, is on-line,e.g., by sampling a portion of the EUV output, e.g., using a pickoffmirror or sampling “uncollected” EUV light, and/or may operate while thedownstream tool, e.g., photolithography scanner, is off-line, forexample, by measuring the entire EUV output of the EUV light source 20.

As further shown in FIG. 1, the EUV light source 20 may include adroplet control system 90, operable in response to a signal, which insome implementations may include the droplet error described above, orsome quantity derived therefrom the controller 60, to e.g., modify therelease point of the target material from a droplet source 92 and/ormodify droplet formation timing, to correct for errors in the dropletsarriving at the desired irradiation region and/or synchronize thegeneration of droplets with the pulsed laser system 22.

More details regarding various droplet dispenser configurations andtheir relative advantages may be found in U.S. patent application Ser.No. 11/827,803 filed on Jul. 13, 2007, now U.S. Pat. No. 7,897,947,issued on Mar. 3, 2011, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCEHAVING A DROPLET STREAM PRODUCED USING A MODULATED DISTURBANCE WAVE,U.S. patent application Ser. No. 11/358,988 filed on Feb. 21, 2006, andpublished on Nov. 16, 2006, as US2006/0255298-A1, entitled LASERPRODUCED PLASMA EUV LIGHT SOURCE WITH PRE-PULSE; U.S. patent applicationSer. No. 11/067,124 filed on Feb. 25, 2005, now U.S. Pat. No. 7,405,416,issued on Jul. 29, 2008, entitled METHOD AND APPARATUS FOR EUV PLASMASOURCE TARGET DELIVERY, U.S. patent application Ser. No. 11/174,443filed on Jun. 29, 2005, now U.S. Pat. No. 7,372,056, issued on May 13,2008, entitled LPP EUV PLASMA SOURCE MATERIAL TARGET DELIVERY SYSTEM,the contents of each of which are hereby incorporated by reference.

Referring now to FIG. 2, an apparatus for in-situ monitoring of anellipsoidal EUV mirror 30 to determine a degree of optical degradationis shown. As shown, the monitoring apparatus may include a light source100 positioned to direct light toward the reflective surface of themirror 30 to illuminate the mirror surface. Typically, the light sourceprovides light which is outside the EUV spectrum (i.e. outside of thewavelength range 1 nm-100 nm. In one setup, the light source may providevisible light. In some cases, it may be favorable to use a point sourceof light, for example, the point source may be a light emitting diode(LED), e.g., ˜1 mm diameter LED with a relatively small emission areaand a relatively large divergent light emission may be used.Alternatively, a larger light source positioned behind an aperture forreducing the emission region, e.g., a relatively small aperture, may beused.

For the ellipsoidal shaped mirror, the light source 100 may bepositioned at one of the foci, such as the close (or primary) focus 28shown in FIG. 2, thus, generating a cone of reflected light 102 havingan apex at the (far or secondary) focus 40, as shown. Also shown, themonitoring apparatus may include a detector 104 which may include, forexample, a screen, e.g. a white screen, for producing an image of thereflected light together with a CCD camera and optional lens forrecording the reflected light distribution. Alternatively, a CCD cameramay be placed in the light cone after the intermediate focus 40.Although the screen is shown positioned downstream of the secondaryfocus 40, it may, as an alternative, be positioned between the primaryand secondary foci 28, 40. Other suitable detectors may include, but arenot necessarily limited to fluorescent screens, photodiode arrays andother optical cameras.

In use, the EUV source 20 shown in FIG. 1 may be operated for apre-determined number of pulses. The EUV light source 20 may then beshut down and the vacuum chamber 26 opened. Once opened, the lightsource 100 and detector 104 may be positioned at their respectivelocations. In an alternate arrangement, a positioning system (notshown), may be installed in the chamber 26 allowing the light source 100and detector 104 to be positioned without breaking the high vacuum inthe chamber 26. In either case, the monitoring apparatus may be used todetermine a degree of mirror optical degradation without requiring themirror to be moved and without affecting the mirror's alignment. Oncethe light source 100 and detector 104 have been properly positioned, animage of the reflected light may be obtained by the detector andcompared to previously obtained data (i.e. a previously measured (i.e.empirically derived) or calculated relationship between mirrordegradation and light reflectivity. For example, synchrotron radiationmay be used to determine EUV reflectivity to establish an empiricalrelationship between EUV reflectivity and reflectivity of non-EUV light.Typically, an increase of mirror surface micro-roughness, e.g. caused byion/particle impacts and/ or material deposits, e.g. micro-droplets,etc., will result in a corresponding decrease in specularly reflectedlight. If desired, the optical degradation measurement may then be usedto estimate EUV reflectivity.

Alternatively, or in addition to using the monitoring apparatus after apredetermine number of EUV light output pulses, the monitoring apparatusmay be used to diagnose an out-of-spec (or near out-of-spec) EUV lightsource, for example, an EUV light source having non-spec EUV outputintensity, bandwidth, angular uniformity, etc. For a typical EUV lightsource, a number of factors may affect EUV light output such as mirrorreflectivity, input laser energy and characteristics, droplet size,droplet-laser pulse interaction, etc. With this large number ofvariables, it may be difficult to isolate which factor(s) is causing anout-of-spec EUV output simply by making adjustments to the various lightsource components. With this in mind, the mirror monitoring apparatusdescribed herein allows an optical degradation measurement to beperformed without removing the mirror from the light source and withoutthe need to generate EUV light to perform the measurement.

FIG. 3 shows an alternative arrangement in which a light source 100(e.g., a source emitting light outside the EUV spectrum as describedabove) may be positioned at one of the foci of an ellipsoidal mirror 30,such as the (far or secondary) focus 40 shown in FIG. 3, and oriented toilluminate the reflective surface of the mirror 30, thus, generating acone of reflected light 102 having an apex at the close (or primary)focus 28, as shown. Also shown, the monitoring apparatus may include anoptic, e.g. a ninety degree turning mirror, positioned at or near theprimary focus 28 and oriented to direct light from the primary focus 28to a detector 104 (as described above). Similarly, the light source maybe imaged to the secondary focus 40 by suitable optics and ninety degreeturning mirror. Monitoring of optical degradation can thus be madethrough windows of the source chamber without breaking the vacuum of thesystem.

FIG. 4 shows another arrangement in which diffuse reflections, e.g.scattered light, may be evaluated (alone or together with the specularreflections described above) to determine a degree of mirror opticaldegradation. Typically, an increase of mirror surface microroughness,e.g. caused by ion/particle impacts and/or material deposits, e.g.microdroplets, etc., will result in a corresponding increase in theamount of diffuse reflected light. As shown, a light source 100 may bepositioned or projected to illuminate a portion or all of the EUV mirror30 surface resulting in specular reflections related to the generalfigure of the mirror surface and diffuse reflections related tosmall-scale surface roughness, e.g. caused by ion/particle impacts and/or material deposits. For the particular example shown in FIG. 3, thelight source 100 is shown positioned at the primary focus 28 of anellipsoidal mirror 30, thus, generating a cone of specularly reflectedlight 102 having an apex at the secondary focus 40, as shown. Alsoshown, the monitoring apparatus may include a detector 104, e.g. CCDcamera as described above, positioned at an oblique angle relative tothe illuminating rays to measure the amount of a diffuse reflection. Themeasured data may then be compared to previously obtained data (i.e., apreviously measured (i.e., empirically derived) or calculatedrelationship between mirror degradation and diffuse light reflectivity.

FIG. 5 shows another arrangement in which diffuse reflections, e.g.scattered light, may be evaluated (alone or together with the specularreflections described above) to determine a degree of mirror opticaldegradation. As shown, a laser source 150 may be positioned to direct anincident laser beam 152 to a relatively small surface location 153 ofellipsoidal EUV mirror 30 surface (having foci 28, 40). As shown, thebeam is specularly reflected therefrom as reflected beam 154. Detector156 is positioned to receive scattered light at a pre-selected anglerelative to the angle of incidence, and in some cases, may be moveable,e.g. along arrow 158, to measure scattered light at a plurality ofangles, relative to the angle of incidence. Scattered light from severalplaces on the collector mirror 30 surface may be evaluated, or, ifdesired, the entire surface may be scanned, e.g. raster scan, with thelaser beam. The measured data may then be compared to previouslyobtained data (i.e. a previously measured (i.e. empirically derived) orcalculated relationship between mirror degradation and diffuse lightreflectivity. The incident and the scattered light can be propagatedthrough chamber windows. Thus, such measurements of scattered light canbe made without breaking the vacuum of the system. Although the abovedescription (i.e. description of FIGS. 2-5) has been made with referenceto ellipsoidal collector mirrors, it is to be appreciated that theteachings described above extend beyond collector mirrors, and inparticular, beyond near-normal incidence ellipsoidal mirrors, toinclude, but not necessarily limited to, flat mirrors, sphericalmirrors, aspherics, parabolic mirrors, grazing angle incidence mirrorsand so-called ring field optics/collector mirrors.

FIG. 6 illustrates a portion of a metrology device 200 for measuring acharacteristic of EUV radiation. As shown, the device 200 may include adetecting element 202 and a narrowband EUV transmission filter 204. Forexample, the detecting element 202 may be a fluorescence converter,e.g., having a CE:YAG crystal, for measuring, for example, angularintensity distribution of EUV exiting the EUV light source, or thedetecting element 202 may be a photodiode for measuring EUV intensity.For the device 200, the filter 204 may be a coating (having one or morelayers) deposited to overlay, and in some cases contact, an operablesurface of the detecting element 202. Alternatively, or in addition tothe deposited coating, the filter 204 may consist of one or morenon-deposited films/foils that are positioned along the EUV light pathand in front of the detector element 202. In this regard, severalfilters are disclosed varying in material composition and thickness,with each filter having an EUV transmission bandwidth and peaktransmission. Typically, the metrology device 200 may be used downstreamof one or more multi-layer mirrors, e.g. Mo/Si mirrors, which reflectlight having a relatively small EUV bandwidth and an intensity peak atabout 13.5 nm. However, in the absence of suitable filters, metrologydetectors measuring the output of an EUV light source may also exposed(undesirably) to light at other wavelengths, e.g. light in the visible,IR and UV spectrums as well as out-of-band EUV radiation. Moreover, ascurrently contemplated, light exiting an EUV light source may bereflected from a number of Mo/Si mirrors, with each mirror filtering theEUV light source output before the EUV light interacts with a wafer.Thus, it may be desirable to simulate, via filter(s), the EUV lightreaching the wafer when performing metrology on the EUV light to sourceoutput. Heretofore, it has been suggested to use Zr, which has arelatively wide bandwidth around 13.5 nm, or, Si, which is peaked near12.5 nm, due to location of Si-absorption edge, or combinations thereof.

Referring now to FIG. 7A, several calculated plots of transmissionintensity (normalized) versus wavelength in nanometers are shown, withplot 300 corresponding to a silicon nitride (Si₃N₄) filter having athickness of 200 nm. As shown, the Si₃N₄ filter has a peak transmissionat a wavelength of about 12.5 nm. As compared to Si which has beenpreviously suggested for EUV filtration, Si₃N₄ foils have higher tensilestrength and are also more inert and resistant to chemically aggressiveenvironments. Also, silicon nitride may be combined with a transitionmetal to produce a bandwidth less than that obtained when only usingsilicon nitride. FIG. 7A shows calculated plots of transmissionintensity (normalized) versus wavelength for two siliconnitride/transition metal combinations. In particular, plot 302corresponds to a filter having silicon nitride (Si₃N₄) having athickness of 200 nm and Palladium (Pd) at a thickness of 50 nm, and plot304 corresponds to a filter having silicon nitride (Si₃N₄) having athickness of 200 nm and Ruthenium (Ru) at a thickness of 50 nm. For bothplots 302, 304, the transmission peak is near 12.5 nm and the bandwidthis narrower than plot 300 corresponding to a filter having only siliconnitride. Specifically, the full-width half-max (FWHM) bandwidth forSi₃N₄/Pd (plot 302) is <1 nm and the FWHM bandwidth for Si₃N₄/Ru (plot304) is near 1.5 nm.

Referring now to FIGS. 7B, 7C and 7D, several calculated plots oftransmission intensity versus wavelength in nanometers are shown, withplot 400 (FIG. 7B) corresponding to a de-enriched Uranium filter havinga thickness of 0.1 μm, plot 402 (FIG. 7C) corresponding to a de-enrichedUranium filter having a thickness of 0.2 μm plot 404 (FIG. 7D)corresponding to a de-enriched Uranium filter having a thickness of 0.3μm. As shown, the peak of EUV transmission is near 13.3 nm and the FWHMbandwidth is between about ˜2 nm and ˜1 nm, depending on the thicknessof the filter. Note: the thicker the filter, the narrower the bandwidthand the lower the transmission. It can also be seen that thetransmission to EUV radiation near the peak is between 40% and 10%,depending on the thickness of the filter.

FIG. 7E shows a calculated plot (plot 406) of transmission intensityversus wavelength in nanometers for a filter having about 0.2 μm thickUranium layer and about 50 nm thick Ru layer. It can be seen that thebandwidth is narrower than Uranium filters (FIGS. 7B-D) and the peaktransmission is near 10%. Another suitable filter may be made of a Mo/Sior Zr/Si transmission multilayer consisting of 20 to 40 bilayers. Thetransmission is near 2% and the bandwidth is about 0.4 nm and thebilayer period is about 7.0 nm in the case of Mo/Si. In the case of aZr/Si transmission multilayer, using 20 bilayers with e.g. a siliconlayer thickness of 4.0 nm and a Zr layer thickness of 1.75 nm, atransmission of almost 80% can be obtained. However, the bandwidth ismore than 7 nm wide (full-width at half-maximum) in this case.

FIG. 8A illustrates an embodiment of a backside heater for an EUVreflective mirror, i.e. a heater positioned on the side opposite thereflective surface of the mirror, for controlling the temperature of thereflective surface of a mirror, such as a collector mirror 30. In oneapplication, the heater may be employed to control the mirror's surfacetemperature, and thus, the etch rate for an BUY light source whichemploys an etchant to react with plasma generated debris that hasdeposited on the mirror's surface. Typically, the etch rate may bedependent on temperature. For example, the rate of removal of Tindeposited using HBr and/or Br₂ etchants has been found to be stronglydependent on temperature in the range of 150-400° C. As detailed furtherbelow, the backside heater may be configured to heat different zones ofthe surface to different temperatures to maintain a uniform temperatureon the mirror's reflective surface, or, to provide higher surfacetemperatures at zone(s) where more debris is deposited, thus increasingthe etch rate for these zone(s). For example, for an ellipsoidalcollector mirror with plasma generation at the near focus, some zones ofthe mirror will be closer to the plasma, and thus, may be heated moredue to the plasma than other zones. For this case, if desired, thebackside heater may employ differential heating to establish a uniformtemperature at the mirror's reflective surface.

Collector mirror lifetime may play a dominant role in the overall costof an EUV light source. Thus, it may be desirable to employ collectormirror components such as heating systems having relatively long servicelives. In this regard, in some arrangements, portions or all of theheating system may be exposed to etchants such as HBr/Br₂ as well aselevated temperatures. Moreover, for some arrangements, heating systemcomponents may be in fluid communication with the operable portion ofthe EMT light source. For these arrangements, it may be desirable to usematerials which do not create contaminants which may deposit on theoperable surfaces of optics and/or absorb EUV light. Coating of mirrorfixtures and mirror surfaces that are not used for reflection with alayer of one to several 100 nm thickness of a non-reactive compound likesilicon nitride or silicon oxide can be applied to avoid reaction withthe etchant and prevent surface erosion.

FIG. 8A shows an arrangement of a backside heater in which a conductivecoating 500 such as molybdenum has been deposited onto the backside 502of the collector substrate in a pre-selected pattern consisting of twocircuits, with each circuit having a pair of terminals. Although twocircuits are shown in FIG. 8A, it is to be appreciated that more thantwo and as few as one circuit may be used.

For example, the coating may be applied directly onto the substrateusing physical vapor deposition, chemical vapor deposition,flame-spraying electroplating or a combination thereof. Directapplication of the substrate provides a good heat contact of theconductive material and the substrate. The substrate may be composed of,for example, SiC, polycrystalline silicon or single crystal silicon. Inmost cases, it may be desirable to match the thermal expansioncoefficient of the conductive coating material and substrate, forexample, to prevent cracking, peeling, etc., of the coating. In thisregard, Mo and SiC have relatively close thermal expansion coefficients.

The SiC substrate has a fairly high surface resistivity of ˜1 kΩ/cm-1000kΩ/cm, depending on surface purity. The resistance along the Mo backsideheater is less than 1 Ω/cm, thus the heating current will flow almostentirely through the Mo heating loops.

FIG. 8A also shows that the backside heater may include a system 504,e.g. one or more regulatable current source(s), generating controllableelectrical currents in the coating 500 to heat the collector mirror. Forthe deposit backside heater shown in FIG. 8A, the amount of heatgenerated may be selectively varied from one zone to another on thecollector surface in several ways. For example, the coating thicknessand/or the coating width, “w” and/or surface coverage (e.g. thepercentage of surface within a zone covered by conductor) and/or coatingconductivity may be varied to establish differential heating.Alternatively, or in addition to the variations described above,multiple circuits may be employed, each having a different patternsand/or each being connected to an independent current source.

FIG. 8B shows an arrangement of a backside heater in which a conductivecoating 600 such as molybdenum has been deposited (e.g. as describedabove) onto the backside 602 of the collector substrate in apre-selected pattern consisting of six loops, with each loop forming aclosed electrical pathway. Although six loops are shown in FIG. 8B, itis to be appreciated that more than six and as few as one loop may beused. For the backside heater shown in FIG. 8B, a system 604 may beprovided selectively establishing eddy currents in each loop to heat thecollector mirror surface. For example, the system 604 may consist of oneor more inductors positioned behind the collector mirror. In anothersetup, microwave radiators may be used. If desired, the system 604 maybe configured to establish eddy currents in each loop independently,thereby allowing different zones to be heated independently.

For the deposited backside heater shown in FIG. 8B, the amount of heatgenerated may be selectively varied from one zone to another on thecollector surface in several ways. For example, the coating thicknessand/or the coating width, “w” and/or surface coverage (e.g. thepercentage of surface within a zone covered by conductor) and I orcoating conductivity may be varied to establish differential heating.Alternatively, or in addition to the variations described above,multiple loops may be employed, each having a different patterns and/oreach being energized by an independent inductor.

FIG. 8C shows another embodiment in which a portion or all of the mirrorsubstrate 700 may be doped with a conductive material 702, for example,a SiC substrate doped with graphite. With this structural arrangement,the substrate may be selectively heated by exposing the doped portionsto RF or microwave radiation. The amount of heat generated may beselectively varied from one zone to another on the collector surface byvarying the doping levels within the substrate and/or varying thestrength of the radiation reaching a particular zone.

FIG. 9A shows a sectional view of a mirror, e.g., a near-normalincidence EUV collector mirror, having a substrate 800, intermediatelayer 802, and multi-layer mirror coating 804. FIG. 9A illustrates amethod for manufacturing and/or refurbishing an EUV mirror in which ametallic substrate, e.g. Ni, Al, Ti or materials like invar, covar,monel, hastelloy, nickel, inconel, titanium, nickel-phosphiteplated/coated aluminum or nickel-phosphite plated/coated invar, or asemi-conductor material like silicon, e.g., single- or multi-crystallinesilicon is diamond turned to produce an exposed surface having thegeneral figure of the final optic, e.g., ellipsoidal, spherical,parabolic, etc., and having a surface roughness of about 2-10 nm. Next,an intermediate layer 802 is deposited which may be a so-called“smoothening” layer deposited using a physical vapor depositiontechnique to reduce surface roughness which can affect MLM performance.For example, the physical vapor deposition technique may be selectedfrom the group of techniques consisting of ion beam sputter deposition,electron beam deposition, physical vapor deposition, magnetronsputtering and combinations thereof, The smoothing material may be anamorphous material and/or may be selected from the group of materialsconsisting of Si, C, Si₃N₄, B₄C, SiC and Cr. The smoothing material maybe deposited using highly energetic deposition conditions, for example,the deposition conditions may include substrate heating and/or thedeposition conditions may include increasing particle energy duringdeposition. In some cases, the smoothing layer may overlay and contactthe metallic substrate and may have a thickness in the range of about 3nm to 100 nm. FIG. 9A shows that a multi-layer mirror coating 804, e.g.a coating having about 30-90 Mo/Si bilayers may be deposited to overlaythe intermediate layer 802.

FIG. 9B shows a sectional view of a mirror, e.g., a near-normalincidence EUV collector mirror, having a substrate 850, firstintermediate layer 852, second intermediate layer 854 and multi-layermirror coating 856. FIG. 9A illustrates a method for initial manufactureof an EUV mirror in which a substrate, e.g. made of a material like SiC,polycrystalline silicon, single-crystal silicon, Ni, Al, Ti or materialslike invar, covar, monel, hastelloy, nickel, Inconel, titanium,nickel-phosphite plated/coated aluminum is processed by a techniquesuitable for the shaping the substrate material, e.g., diamond turning,grinding, lapping and polishing, etc., to produce an exposed surfacehaving the general figure of the final optic, e.g., ellipsoidal,spherical, parabolic, etc.

Next, intermediate layers 852, 854 are deposited with one of the layersbeing so-called “smoothening” layer and the other being a so-called“stop” layer. The “smoothening” layer may be deposited after the “stop”layer, or vice-versa. Each of these layers may be deposited using aphysical vapor deposition technique as described above. As describedabove, the smoothening material may be an amorphous material and/or maybe selected from the group of materials consisting of Si, C, Si₃N₄, B₄C,SiC, ZrN, Zr and Cr, and may be deposited using highly energeticdeposition conditions to a thickness in the range of about 3 nm to 100nm.

Two different types of stop layers are described herein. In onearrangement, a relatively thin “etch” stop layer, (e.g., 1-100 nm and insome cases 5-20 nm) may be used to allow the MLM coating to be removedvia etching during a refurbishment procedure, while leaving the etchstop layer. For example, suitable etching techniques may include, butare not necessarily limited to chemical wet etching, dry plasma etchingor reactive ion etching. Typically, the etch stop layer material isselected to have a substantially different etch sensitivity than themulti-layer mirror coating for at least one etchant. Suitable etch stopmaterials may include, but are not necessarily limited to, Si, B₄C,oxides such as TiO₂, nitrides such as ZrN, SiC, Zr and Cr.

A second type of stop layer is disclosed herein in which a relativelythick stop layer (e.g. 3 μm-20 μm, and in some cases 5 μm to 15 μm) maybe used to allow the MLM coating to be removed via diamond turningduring a refurbishment procedure, while leaving the etch stop layer.Suitable materials for this second type of stop layer may include, butare not necessarily limited to, Si, B₄C, oxides such as TiO₂, SiC, Zr,Cr and nitrides such as ZrN.

FIG. 9B also shows that a multi-layer mirror coating 856, e.g. a coatinghaving about 30-90 Mo/Si bilayers may be deposited to overlay theintermediate layers 852, 854.

Refurbishment of the mirror shown in FIG. 9B may be performed byremoving the MLM coating 856, either by diamond turning or etching asdescribed above to expose the stop layer and thereafter depositing asmoothening layer on the exposed stop layer followed by depositing a newMLM coating. For the refurbishment, the smoothening layer may be anamorphous material and/or may be selected from the group of materialsconsisting of Si, C, Si₃N₄, B₄C, SiC, ZrN, Zr and Cr, and may bedeposited using highly energetic deposition conditions to a thickness inthe range of about 3 nm to 100 nm.

In addition to the intermediate layers described above, one or morebarrier layer to prevent diffusion of one layer into another may beprovided, and in some cases a release layer such as chromium orzirconium may be provided overlaying the stop layer to facilitate MLMcoating removal. These additional layers may be deposited during initialfabrication and, in some cases during refurbishment. For example, thebarrier material may be selected from the group of materials consistingof MoSi₂, Si₃N₄, B₄C, SiC, ZrN, Zr and Cr, and may be positionedsomewhere between the substrate and MLM. In some cases, a barrier layermay be provided between the substrate and stop layer and/or between thestop layer and MLM coating.

FIG. 10 shows a sectional view of a mirror, e.g., a near-normalincidence EUV collector mirror 930, having a substrate 902, multi-layercoating stacks 904 a-e and stop layers 906 a-d, with each stop layers906 a-d interposed between a pair of multi-layer coating stacks 904 a-e.For example, each multi-layer coating stack 904 a-e may include aboutforty to two hundred bi-layers, and more typically eighty to one hundredtwenty bi-layers, with each bi-layer having a layer of relatively highrefractive index material and a layer of relatively low refractive indexmaterial. In one arrangement, each bi-layer may include a layer of Moand a layer of Si. For the mirror 930, each stop layer 906 a-d may beformed of a material selected from the group of materials consisting ofSi₃N₄, SiB₆, SiC, C, Cr, B₄C, Mo₂C, SiO₂, ZrB₂, YB₆ and MoSi₂. For themirror 930, the stop layer may have a thickness selected to maintain theperiodicity of the mirror from one adjacent multi-layer coating stack tothe other adjacent multi-layer coating stack. For example, the stoplayer can be made at a thickness so that it replaces one layer in themultilayer stack that would otherwise be a silicon layer. (e.g., about 4nm thick for near-normal angle of incidence). In some cases, the stoplayer material may be selecting in conjunction with a suitable etchantsuch that the etchant has a relatively high etch rate for themulti-layer coating stack materials, e.g. Mo and Si, and a relativelyhigh etch rate for the stop layer. Etchants may, for example, beselected from the group of materials consisting of Cl₂, HCl, CF₄, andmixtures thereof.

FIG. 11 shows a sectional view illustrating a mirror 930′, e.g., anear-normal incidence EUV collector mirror, having a substrate 952 andmulti-layer coating stacks 954 a,b that are separated by a stop layer956 (note: the series of dots indicates that the multilayer stack mayrepeat, as necessary, to establish the desired number of layers). Asfurther shown, each multi-layer coating stack 954 a,b may have aplurality of relatively high refractive index layers, such as layer 958,a plurality of relatively low refractive index layers, such as layer960, and a plurality of diffusion barrier layers, such as layers 962 a,bseparating relatively high index layers from the relatively low indexlayers. In one arrangement, each “bi-layer” may include a layer of Mo, alayer of Si, and two diffusion barrier layers. For example, thediffusion barrier layers may be, for example, silicon nitride, carbon orB₄C and each stop layer 956 may be formed of a material selected fromthe group of materials consisting of Si₃N₄, SiB₆, SiC, C, Cr, B₄C, Mo₂C,SiO₂, ZrB₂, YB₆ and MoSi₂.

In use, the mirrors 930, 930′ may be disposed in chamber, e.g. chamber26 shown in FIG. 1, and used to reflect EUV light produced by an EUVlight emitting plasma. As described earlier, the plasma may generatedebris including energetic ions which may reach and degrade the exposedsurface, and, more specifically, the multi-layer coating stack nearestthe surface. In most cases, the degradation may not result in uniformwear/removal. Instead, the multi-layer coating stack may be removedunevenly resulting in surface roughness which may decrease the in-bandreflectivity of the mirror. In some cases, terraces and mesas maydevelop.

Once a predetermined amount of coating stack removal and/or apredetermined increase in mirror surface roughness and/or apredetermined decrease in EUV in-band reflectivity has occurred, theremaining portion of the multi-layer coating stack nearest the surfacemay be etched away, e.g. using an etchant having a relatively high etchrate for the multi-layer coating stack materials, e.g., Mo and Si, and arelatively high etch rate for the stop layer. Once the remaining portionof the multi-layer coating stack nearest the surface is removed, etchingmay be discontinued until needed again to etch the next multi-layercoating stack, and continued use of the mirror 930, 930′ in the lightsource may occur, with the stop layer acting as a capping layer.

In some cases, etching may be performed in-situ, e.g., with the mirror930, 930′ positioned in the chamber, and in some cases, an etchant maybe introduced into the chamber during EUV light production.Alternatively, etching may be performed during periods of scheduledmaintenance and/or after removing the mirror 930, 930′ positioned in thechamber from the chamber. As indicated above, etchants may, for example,be selected from the group of materials consisting of Cl₂, HCl, CE₄, andmixtures thereof.

While the particular embodiment(s) described and illustrated in thispatent application in the detail required to satisfy 35 U.S.C. §112 arefully capable of attaining one or more of the above-described purposesfor, problems to be solved by, or any other reasons for or objects ofthe embodiment(s) above described, it is to be understood by thoseskilled in the art that the above-described embodiment(s) are merelyexemplary, illustrative and representative of the subject matter whichis broadly contemplated by the present application. Reference to anelement in the following Claims in the singular is not intended to meannor shall it mean in interpreting such Claim element “one and only one”unless explicitly so stated, but rather “one or more”. All structuraland functional equivalents to any of the elements of the above-describedembodiment(s) that are known or later come to be known to those ofordinary skill in the art are expressly incorporated herein by toreference and are intended to be encompassed by the present Claims. Anyterm used in the Specification and/or in the Claims and expressly givena meaning in the Specification and/or Claims in the present Applicationshall have that meaning, regardless of any dictionary or other commonlyused meaning for such a term. It is not intended or necessary for adevice or method discussed in the Specification as an embodiment toaddress or solve each and every problem discussed in this Application,for it to be encompassed by the present Claims. No element, component,or method step in the present disclosure is intended to be dedicated tothe public regardless of whether the element, component, or method stepis explicitly recited in the Claims. No claim element in the appendedClaims is to be construed under the provisions of 35 U.S.C. §112, sixthparagraph, unless the element is expressly recited using the phrase“means for” or, in the case of a method claim, the element is recited asa “step” instead of an “act”.

1. An EUV light producing system having at least one mirror disposed ina chamber and a system for monitoring an optical condition of saidmirror, comprising: a light source irradiating at least a portion ofsaid mirror with first light having a spectrum outside of EUV lightspectrum while the mirror is in an operational position characteristicof EUV light production in said chamber; and a detector arrangementmeasuring an optical characteristic of a portion of second light, saidsecond light representing at least a portion of said first lightreflected from said portion of said mirror, the detector arrangementgenerating information pertaining to said optical condition.
 2. The EUVlight producing system of claim 1 wherein said mirror has a primaryfocus and a secondary focus, said light source being disposed at saidsecondary focus, a distance between said secondary focus and said mirrorbeing larger than a distance between said primary focus and said mirror.3. The EUV light producing system of claim 1 wherein said mirror has aprimary focus and a secondary focus, said light source being disposed atsaid primary focus, a distance between said secondary focus and saidmirror being larger than a distance between said primary focus and saidmirror.
 4. The EUV light producing system of claim 3 wherein saiddetector arrangement is positioned outside a cone of specularlyreflected light from said mirror, said second light representingdiffused reflection of said first light after said first light isreflected from said portion of said mirror.
 5. The EUV light producingsystem of claim 1 wherein said first light represents light in thevisible light spectrum.
 6. The EUV light producing system of claim 1wherein said mirror has a primary focus and a secondary focus, saidlight source being disposed at one of said primary focus and saidsecondary focus, a distance between said secondary focus and said mirrorbeing larger than a distance between said primary focus and said mirror,wherein a distance between said detector arrangement and said mirror isgreater than a distance between said primary focus and said mirror. 7.The EUV light producing system of claim 6 wherein said detectorarrangement includes at least a screen for receiving said second light.8. The EUV light producing system of claim 6 wherein said detectorarrangement includes at least a charged coupled device (CCD) forreceiving said second light.
 9. The EUV light producing system of claim6 wherein said distance between said detector arrangement and saidmirror is less than a distance between said secondary focus and saidmirror.
 10. The EUV light producing system of claim 1 further comprisinga positioning system configured to moving said light source into anirradiating position said chamber for said irradiating said at least aportion of said mirror, wherein said light source is not positioned bysaid positioning system at said irradiating position when said chamberis employed for generating said EUV light.
 11. The EUV light producingsystem of claim 1 wherein said mirror represents a collector mirror. 12.An EUV light producing system having at least a first mirror disposed ina chamber and a system for monitoring an optical condition of said firstmirror, comprising: a light source irradiating at least a portion ofsaid first mirror with first light having a spectrum outside of EUVlight spectrum while said first mirror is in an operational positioncharacteristic of EUV light production in said chamber; a detectorarrangement; and a second mirror for receiving second light andreflecting said second light onto said detector arrangement, said secondlight representing at least a portion of said first light reflected fromsaid portion of said first mirror, said detector arrangement generatinginformation pertaining to said optical condition.
 13. The EUV lightproducing system of claim 12 wherein said second mirror represents aturning mirror and said first mirror represents a collector mirror. 14.The EUV light producing system of claim 12 wherein said first mirror hasa primary focus and a secondary focus, said light source being disposedat said secondary focus, said second mirror being disposed at saidprimary focus, a distance between said secondary focus and said mirrorbeing larger than a distance between said primary focus and said mirror.15. The EUV light producing system of claim 12 wherein said detectorarrangement is positioned outside a cone of specularly reflected lightfrom said first mirror, said second light representing diffusedreflection of said first light after said first light is reflected fromsaid portion of said first mirror.
 16. The EUV light producing system ofclaim 12 wherein said detector arrangement includes at least a chargedcoupled device (CCD) for receiving said second light from said secondmirror.
 17. An EUV light producing system, comprising: mirror means forreflecting light, said mirror means having a first side and a secondside opposite said first side; a laser light source disposed toward saidfirst side of said mirror means, wherein a primary focus and a secondaryfocus are disposed toward said second side of said mirror means, saidlaser light source configured to irradiate targets at said primary focusthrough an aperture in said mirror means to generate EUV light which isthen reflected by said mirror means to said secondary focus; lightirradiating means disposed toward said second side of said mirror meansfor irradiating at least a portion of said mirror means with first lightwhile the mirror means is in an operational position characteristic ofEUV light production in said chamber; and detecting means for measuringan optical characteristic of a portion of second light, said secondlight representing at least a portion of said first light reflected fromsaid portion of said mirror means, the detecting means generatinginformation pertaining to an optical condition of said mirror means. 18.The EUV light producing system of claim 17 wherein said lightirradiating means is disposed at said secondary focus, a distancebetween said secondary focus and said mirror being larger than adistance between said primary focus and said mirror.
 19. The EUV lightproducing system of claim 17 wherein said light irradiating means isdisposed at said primary focus, a distance between said secondary focusand said mirror being larger than a distance between said primary focusand said mirror.
 20. The EUV light producing system of claim 17 whereinsaid light irradiating means represents another laser light source andsaid first light is another laser light.
 21. The EUV light producingsystem of claim 20 wherein said second light represents scattered light,said detecting means being movable to measure said second light at aplurality of angles.
 22. The EUV light producing system of claim 20wherein said light irradiating means is configured to scan a surface ofsaid mirror means.
 23. The EUV light producing system of claim 20further comprising a vacuum chamber, wherein said mirror means and saidlaser light source are disposed within said chamber and wherein at leastone of said light irradiating means and said detecting means is disposedoutside of said chamber.